perksy
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posted on 5/12/10 at 05:27 PM |
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Battery Voltage Drop - How Much ?
Quick question please lads
What sort of Voltage drop should you see when a Multimeter is connected between one of the Battery cables and its Battery Terminal ?
ie how many Amps OR Millivolts ?
This would be on a modern petrol engined car with an ECU
Cheers
Perksy
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matt_gsxr
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posted on 5/12/10 at 05:36 PM |
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Under what conditions?
Engine off, cranking, engine running
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BenB
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posted on 5/12/10 at 05:48 PM |
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If you're connecting in between the cable and the connector (ie in series) then you're measuring AMPs. If you're connecting it in
parallel then you're measuring volts.
Either way it depends on the situation as already said.
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perksy
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posted on 5/12/10 at 05:50 PM |
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Sorry
This would be with the Engine turned off, no lights or radio on etc
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RazMan
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posted on 5/12/10 at 05:53 PM |
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Quiescent (open-circuit) voltage at full charge: 12.6 V
Fully discharged: 11.8 V
Charge with 13.2–14.4 V
Gassing voltage: 14.4 V
Continuous-preservation charge with max. 13.2 V
After full charge the terminal voltage will drop quickly to 13.2 V and then slowly to 12.6 V
Open circuit voltage is measured 12 hours after charging to allow surface charge to dissipate and enable a more accurate reading.
All voltages are at 20 °C, and must be adjusted -0.022V/°C for temperature changes.
[Edited on 5-12-10 by RazMan]
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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russbost
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posted on 5/12/10 at 06:04 PM |
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Razman, I think you've misunderstood the OP's Q - tho' your voltages are spot on & I'm sure will be useful info for
others.
I think the Q is "if you look at the change in voltage when connecting direct to the battery post as compared with connecting to the
other side of the connector, what difference would you expect" - if that is indeed what the OP meant then the answer is virtually no difference
at all other than tiny fractions of a volt - if there is any significant resistance at the battery connection point (-ve or +ve), which would need to
be there to produce any significant voltage drop, then you are certainly going to have starting/cranking probs amongst others!
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RazMan
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posted on 5/12/10 at 06:20 PM |
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oops! Of course you are right on both counts Russ.
Ignore what I just said then - it all depends on the size of the cable, but as Russ said, you should only see a fraction of a volt drop. So if you are
seeing more than that you have a high resistance somewhere so start checking tightness / cleanliness of earth terminals and connectors.
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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perksy
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posted on 5/12/10 at 06:28 PM |
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quote:
if that is indeed what the OP meant then the answer is virtually no difference at all other than tiny fractions of a volt - if there is any
significant resistance at the battery connection point (-ve or +ve), which would need to be there to produce any significant voltage drop, then you
are certainly going to have starting/cranking probs amongst others!
By tiny fractions of a Volt would that be 0.1 volt or ?
Obviously there's things like the digital clock etc
But just wandered what these should all add up to
Just trying to help a mate out with a Battery that seems to discharge overnight
Thinking it might be the Battery about to Die at the moment as the Alternator seems to be doing its job
Thanks
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russbost
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posted on 5/12/10 at 06:47 PM |
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Ahhhh! Different Q entirely!
You don't want to be looking at voltage drops, look at current drawn. Disconnect one terminal & put a 10A ammeter in series with the
disconnected lead DO NOT under any circumstances forget you've got it on there & try & start the engine or you'll have a fried
ammeter!!! Exact amount of current drawn will vary according to radio/clock/alarm etc but you would expect probs less than 0.3 Amps being constantly
drawn (& imho that would be quite high)
If you're drawing more than that try pulling fuses till the current draw goes away. If it still doesn't try disconnecting the alternator
as you could have a diode leaking current. HTH
I no longer run Furore Products or Furore Cars Ltd, but would still highly recommend them for Acewell dashes, projector headlights, dominator
headlights, indicators, mirrors etc, best prices in the UK! Take a look at http://www.furoreproducts.co.uk/ or find more parts on Ebay, user names
furoreltd & furoreproducts, discounts available for LCB users.
Don't forget Stainless Steel Braided brake hoses, made to your exact requirements in any of around 16 colours.
http://shop.ebay.co.uk/furoreproducts/m.html?_dmd=1&_ipg=50&_sop=12&_rdc=1
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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Daddylonglegs
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posted on 5/12/10 at 07:38 PM |
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Sounds like the battery is on it's last legs to me.
Best thing to do is stick an ammeter in-line with the battery supply cable and see what sort of current it is drawing. Shouldn't draw more than
a few hundred milliamps I wouldn't have thought, even with an alarm fitted.
[Edited on 5/12/10 by Daddylonglegs]
It looks like the Midget is winning at the moment......
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RazMan
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posted on 5/12/10 at 07:44 PM |
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If the battery is going flat overnight then there must be a circuit still live which is taking a significant load - either that or the battery should
have been given the last rights months ago
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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DRC INDY 7
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posted on 5/12/10 at 07:59 PM |
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With no load from the battery to power the systems and ecu that has gone to sleep with an IOD (ignition off draw test) then you are looking at 0.01
amps if you are pulling 1 amp upwards then you need to check each circuit ie pulling the fuses till the amps drop
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perksy
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posted on 12/12/10 at 09:32 PM |
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Thanks lads
It was the draw test info i was after
Appologies, Just re-read my original question and it was the ramblings of a muppet
That'll teach me for posting a question after testing the latest batch of stella artois
Cheers
Perksy
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